ix(intel) vs mlxen(mellanox) 10Gb performance

[Rick Macklem]

Daniel Braniss wrote:
> 
> > On 19 Aug 2015, at 16:00, Rick Macklem <rmacklem at uoguelph.ca> wrote:
> > 
> > Hans Petter Selasky wrote:
> >> On 08/19/15 09:42, Yonghyeon PYUN wrote:
> >>> On Wed, Aug 19, 2015 at 09:00:52AM +0200, Hans Petter Selasky wrote:
> >>>> On 08/18/15 23:54, Rick Macklem wrote:
> >>>>> Ouch! Yes, I now see that the code that counts the # of mbufs is before
> >>>>> the
> >>>>> code that adds the tcp/ip header mbuf.
> >>>>> 
> >>>>> In my opinion, this should be fixed by setting if_hw_tsomaxsegcount to
> >>>>> whatever
> >>>>> the driver provides - 1. It is not the driver's responsibility to know
> >>>>> if
> >>>>> a tcp/ip
> >>>>> header mbuf will be added and is a lot less confusing that expecting
> >>>>> the
> >>>>> driver
> >>>>> author to know to subtract one. (I had mistakenly thought that
> >>>>> tcp_output() had
> >>>>> added the tc/ip header mbuf before the loop that counts mbufs in the
> >>>>> list.
> >>>>> Btw,
> >>>>> this tcp/ip header mbuf also has leading space for the MAC layer
> >>>>> header.)
> >>>>> 
> >>>> 
> >>>> Hi Rick,
> >>>> 
> >>>> Your question is good. With the Mellanox hardware we have separate
> >>>> so-called inline data space for the TCP/IP headers, so if the TCP stack
> >>>> subtracts something, then we would need to add something to the limit,
> >>>> because then the scatter gather list is only used for the data part.
> >>>> 
> >>> 
> >>> I think all drivers in tree don't subtract 1 for
> >>> if_hw_tsomaxsegcount.  Probably touching Mellanox driver would be
> >>> simpler than fixing all other drivers in tree.
> >>> 
> >>>> Maybe it can be controlled by some kind of flag, if all the three TSO
> >>>> limits should include the TCP/IP/ethernet headers too. I'm pretty sure
> >>>> we want both versions.
> >>>> 
> >>> 
> >>> Hmm, I'm afraid it's already complex.  Drivers have to tell almost
> >>> the same information to both bus_dma(9) and network stack.
> >> 
> >> Don't forget that not all drivers in the tree set the TSO limits before
> >> if_attach(), so possibly the subtraction of one TSO fragment needs to go
> >> into ip_output() ....
> >> 
> > Ok, I realized that some drivers may not know the answers before
> > ether_ifattach(),
> > due to the way they are configured/written (I saw the use of
> > if_hw_tsomax_update()
> > in the patch).
> > 
> > If it is subtracted as a part of the assignment to if_hw_tsomaxsegcount in
> > tcp_output()
> > at line#791 in tcp_output() like the following, I don't think it should
> > matter if the
> > values are set before ether_ifattach()?
> > 			/*
> > 			 * Subtract 1 for the tcp/ip header mbuf that
> > 			 * will be prepended to the mbuf chain in this
> > 			 * function in the code below this block.
> > 			 */
> > 			if_hw_tsomaxsegcount = tp->t_tsomaxsegcount - 1;
> > 
Well, you can replace the line in sys/netinet/tcp_output.c that looks like:
                        if_hw_tsomaxsegcount = tp->t_tsomaxsegcount;
with the above line (at line #797 in head).

Any other patch for this will have the same effect, rick

> > I don't have a good solution for the case where a driver doesn't plan on
> > using the
> > tcp/ip header provided by tcp_output() except to say the driver can add one
> > to the
> > setting to compensate for that (and if they fail to do so, it still works,
> > although
> > somewhat suboptimally). When I now read the comment in sys/net/if_var.h it
> > is clear
> > what it means, but for some reason I didn't read it that way before? (I
> > think it was
> > the part that said the driver didn't have to subtract for the headers that
> > confused me?)
> > In any case, we need to try and come up with a clear definition of what
> > they need to
> > be set to.
> > 
> > I can now think of two ways to deal with this:
> > 1 - Leave tcp_output() as is, but provide a macro for the device driver
> > authors to use
> >    that sets if_hw_tsomaxsegcount with a flag for "driver uses tcp/ip
> >    header mbuf",
> >    documenting that this flag should normally be true.
> > OR
> > 2 - Change tcp_output() as above, noting that this is a workaround for
> > confusion w.r.t.
> >    whether or not if_hw_tsomaxsegcount should include the tcp/ip header
> >    mbuf and
> >    update the comment in if_var.h to reflect this. Then drivers that don't
> >    use the
> >    tcp/ip header mbuf can increase their value for if_hw_tsomaxsegcount by
> >    1.
> >    (The comment should also mention that a value of 35 or greater is much
> >    preferred to
> >     32 if the hardware will support that.)
> > 
> > Also, I'd like to apologize for some of my emails getting a little "blunt".
> > I just find
> > it flustrating that this problem is still showing up and is even in 10.2.
> > This is partly
> > my fault for not making it clearer to driver authors what
> > if_hw_tsomaxsegcount should be
> > set to, because I had it incorrect.
> > 
> > Hopefully we can come up with a solution that everyone is comfortable with,
> > rick
> 
> 
> ok guys,
> when you have some code for me to try just let me know.
> 
> danny
> 
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